President Obama and Mitt Romney are locked in intense final preparations for Wednesday's presidential debate in Denver, the first of three scheduled face-offs in the final five weeks of the campaign.

Romney is falling behind in many polls — especially in swing states like Ohio and Florida, which are expected to decide the November election — and many political strategists say his first televised showdown with Obama is one of his last big chances to regain momentum.

New Jersey Gov. Chris Christie (R) says that Romney "hasn't had a good two weeks," but that his fellow Republican will turn the race "upside down" in Colorado. And indeed, history shows that presidential (and vice presidential) debates can be game-changers — for better or worse.